1. Field of the Invention
This invention relates to a semiconductor device and its manufacture, and more particularly to a semiconductor device with a metallic precipitate or precipitates such as a single electron tunneling (SET) device formed in a semiconductor body and including a metallic precipitate at controlled position, and its manufacture.
2. Description of the Related Art
A SET device has two or more electrical contacts coupled by at least one conductive region which allows a current flow by tunneling, wherein the tunneling of a single electron to or from the conductive region changes the energy of the conductive region to such an extent which is comparable or larger than the ambient noise, such as the thermal energy. This is the case in a structure where at least one electrical node (e.g. a metallic region) involved in the tunneling path is very small and hence has a very small capacitance. One important consequence of the electrical characteristic of such a device, for example, is so-called coulomb blockade of tunneling, wherein tunneling is suppressed under certain conditions, such as at low bias in a SET diode.
Usually, an electric current is carried by electrons or holes in a conductive material, and is not allowed to flow in a non-conductive material including insulators and depleted semiconductors. At the interface between a conductive and non-conductive regions, however, wave functions of carriers are not perfectly limited in the conductive region but penetrate from the conductive region into the non-conductive region to some extent. When a pair of conductors sandwich a narrow non-conductive region, the wave functions extending from the pair of conductors may overlap in the non-conductive region. In such a case, a current is allowed to flow through the non-conductive region by tunneling.
Various semiconductor devices can be formed utilizing tunneling phenomenon. When there exists a floating electrical node in a current path, the electrical potential of the node varies depending on the charge stored at the node. The SET device is such a device wherein the potential of such a floating node varies with the tunneling of a single electron or electronic charge. An attractive feature of the SET is that the tunneling current path should be very short. Therefore, the size of the SET device can be made very small, enabling miniaturization of the electronic device.
Conventionally, the SET device has been made utilizing Al/oxide/Al tunnel junctions. A major problem with this method of fabrication is that small area junctions are difficult to fabricate. Since the maximum operating temperature scales inversely with junction capacitance, a large size junction area limits the operation to extremely low temperatures. For realizing a SET device operable at practical temperatures, fabrication of ultra-small SET junctions is required.
It is desired to form ultra-small metal particles embedded in a non-conducting material. When a very thin metal film having a thickness less than 10 nm is deposited, by evaporation or sputtering, for example, it is discontinuous and is distributed randomly. Diameters of metallic regions can be controlled, but the spacing of the metallic particles is random and cannot be controlled.
When GaAs film is grown by molecular beam epitaxy (MBE) at a low substrate temperature such as 200.degree. C., the grown GaAs film includes excess As of about 1.5% (4.times.10.sup.20 cm.sup.-3) in antisite defect and a dilation of lattice of about 0.16%. When such a low temperature (LT) GaAs film is annealed, for example at 600.degree. C., As particles are precipitated and the lattice constant decreases to that of GaAS substrate. The diameter and the spacing of As precipitates can be controlled, for example by the growth temperature and the annealing temperature. Typical precipitates densities are in the range of 10.sup.17 to 10.sup.18 cm.sup.-3, and the spacings between particles are typically 10 to 20 nm. The As precipitates are metallic (or semi-metallic), and the surrounding regions becomes highly resistive. It is considered that the As precipitates and the surrounding GaAs region form a Schottky contact. The depletion of free carriers in the surrounding depleted semiconductor region is believed to be the primary cause of the semi-insulating property of the semiconductor region.
In this specification, the term "metallic" is used to mean metal-like or highly conductive and to cover metal, semi-metal, and also conductive semiconductor, unless otherwise specified.
Precipitate formation is known to occur also in other semiconductor materials. Some examples are As precipitates in GaAs, As precipitates in GaInAs, and P precipitates in InP.
In addition to the formation of randomly distributed As precipitates in GaAs and other materials, precipitates can be formed in planes within an AlGaAs/GaAs laminated heterostructure.
For heterostructures with certain layer parameters, the As precipitates are found to form selectively in the GaAs layers of the heterostructure. This occurs despite the fact that precipitates can form in both LT-GaAs and LT-AlGaAs under other conditions. There is found tendency of precipitate depletion in the AlGaAs and accumulation of precipitates in GaAs near the interfaces. When a thin GaAs layer is sandwiched between a pair of AlGaAs layers, each layer being grown by low temperature MBE, the vertical position of the precipitates can be controlled to be located in the GaAs layer. However, the positions of the As precipitates in GaAs layer cannot be controlled. Therefore, there is no mean to realize a SET device by utilizing such As precipitates.